1. Field of the Invention
This invention relates to an image processing apparatus or system such as a reproduction apparatus, facsimile, and so on.
2. Description of Prior Art
Heretofore, in a reproduction apparatus (throughout the specification it is referred to as a "digital copying machine") which reads an image original by means of a photo-electric transducing element (e.g., a charge-coupled device (CCD)), converts the signal as a read into digital signals, and forms the image for reproduction on a printing device, there has generally been adopted binary expression. This conventional system is to determine picture elements as read out in terms of a threshold value of whether they are black or white, based on which determination, image reproduction is effected on the printing device in the form of black or white dots.
According to this image processing system, density of the half tone level (for example, gray, etc.) is expressed in white or black. Therefore, when a copy is to be made from an image original such as photograph containing various half tones, the resulting copy shows dry and monotonous image quality, i.e., the reproduced image differs in quality from the original photograph.
As a method for solving such problem, there has been used a dither method which is to reproduce half tones by properly selecting the number of dots to be reproduced in a certain definite area. While this method is effective in reproduction of an image containing many half tones such as a silver-salt photograph, etc., when the gradation expression is enhanced (i.e., when 64 gradations, for example, are to be expressed, an area containing 8 dots.times.8 dots is required, and, when 16 gradations are to be reproduced, an area containing therein 4 dots.times.4 dots is necessary), the image quality appears to be very coarse and rough, even though light and shade may come out in the reproduced image. Further, when the gradation is enhanced, reproducibility of thin lines becomes poor. As the consequence of this, there has inevitably taken place the phenomenon such that, in the case of an image original containing letters and characters besides other forms of image, when the reproducibility of these letters and characters is to be increased, the reproducibility of the half tone image becomes poor, and, on the contrary, when reproducibility of the half tone image is to be enhanced, the reproducibility of the characters and letters becomes deteriorated.
There has also been another image processing method, wherein a plurality of CCD are used to read out an image original, and the data as read out are converted into electrical signals. The present inventors have so far proposed a junction (or connection) compensation among the bits relative to automatic connection of the CCD's in the main scanning direction, by placing a marker, or other reference means at an area outside the image original.
In the case of the half tone expression, when the above-mentioned dither method is employed, disturbance takes place in the dither pattern with the simple connection among the bits, whereby unnaturalness occurs in the gradation of the reproduced image at the junction portion. That is to say, the dither method is to express gradations in one brightness by means of an area containing 2.times.2 bits, 4.times.4 bits, 8.times.8 bits, etc., on account of which, where there is a place in the image original in which a subsequent pattern appears before the preceding pattern comes to an end, such junction portion cannot be reproduced with accurate gradation expression, and the resulting image is distorted or degraded in quality.
It has also been practiced heretofore that the dither patterns are permanently stored in the read-only-memory (ROM), from which image data are read out, and the data signals as read out are compared with image signals to thereby obtain binary signals. With this method, however, when it is desired to vary the dither patterns to decrease variations in the reproduction characteristics in light and shade of the image, waste in such reproduced image, pattern data are required to be stored in the ROM for the number of the dither patterns with the consequence that the capacity of the ROM increases. For high speed processing of the read-out data for one line of 55 ns or higher, a bipolar ROM becomes necessary. Consequently, increase in the ROM capacity inevitably leads to increase in the power consumption and the memory cost with the consequence that the patterns to be stored in the ROM should be limited to the minimum necessary, which causes inconvenience in the actual reproduction operations.